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Catch bond drives stator mechanosensitivity in the bacterial flagellar motor

  1. Francesco Pedacia,1
  1. aCentre de Biochimie Structurale (CBS), INSERM, CNRS, Université de Montpellier, 34090 Montpellier, France;
  2. bDepartment of Mathematics, University College London, London WC1E 6BT, United Kingdom;
  3. cBiophysics Graduate Group, University of California, Berkeley, CA 94720;
  4. dDepartment of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 2JD, United Kingdom
  1. Edited by Steven M. Block, Stanford University, Stanford, CA, and approved October 27, 2017 (received for review September 11, 2017)

  1. Fig. 2.

    Stator stoichiometry before and after stall. (A and B) Kernel density estimates (KDEs) of the single-stator torque contribution and the steady-state stoichiometry, respectively, as a function of external viscous load. (C) Temporal evolution of stator stoichiometry of motors driving the different viscous loads (color-coded as in A, B, D, and E). Steady-state rotation of the viscous load corresponds to time <mml:math><mml:mrow><mml:mpadded width="+1.7pt"><mml:mi>t</mml:mi></mml:mpadded><mml:mo><</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math>t<0. The motor is then stalled by the magnetic field for a period of 300 s (indicated by a break in the x axis). At <mml:math><mml:mrow><mml:mi>t</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math>t=0, the motor is released from stall. The thick color-coded line and the colored region are the average and SD of multiple motors. The horizontal gray dashed line indicates the average number of stator units measured for <mml:math><mml:mrow><mml:mpadded width="+1.7pt"><mml:mi>t</mml:mi></mml:mpadded><mml:mo><</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math>t<0 at steady state. The dark dashed line is the fit obtained from Eq. 3 for <mml:math><mml:mrow><mml:mpadded width="+1.7pt"><mml:mi>t</mml:mi></mml:mpadded><mml:mo>></mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math>t>0. <mml:math><mml:msub><mml:mi>N</mml:mi><mml:mrow><mml:mi>S</mml:mi><mml:mi>S</mml:mi></mml:mrow></mml:msub></mml:math>NSS and <mml:math><mml:msub><mml:mi>t</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math>tc in the bottom image indicate the parameters extracted by the exponential fit using Eq. 3. (D) KDE of the number of stator units recruited during stall as a function of external viscous load. (E) Steady-state stoichiometry, <mml:math><mml:msub><mml:mi>N</mml:mi><mml:mrow><mml:mi mathvariant="italic">ss</mml:mi></mml:mrow></mml:msub></mml:math>Nss, as a function of the characteristic relaxation time, <mml:math><mml:msub><mml:mi>t</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math>tc. For comparison, gray lines show the predictions of models, where the variation in <mml:math><mml:msub><mml:mi>K</mml:mi><mml:mi>D</mml:mi></mml:msub></mml:math>KD is due entirely to <mml:math><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi mathvariant="italic">on</mml:mi></mml:mrow></mml:msub></mml:math>kon (dashed), entirely to <mml:math><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi mathvariant="italic">off</mml:mi></mml:mrow></mml:msub></mml:math>koff (solid), or split equally between <mml:math><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi mathvariant="italic">on</mml:mi></mml:mrow></mml:msub></mml:math>kon and <mml:math><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi mathvariant="italic">off</mml:mi></mml:mrow></mml:msub></mml:math>koff (dotted). Number of motors analyzed was 24 for <mml:math><mml:msub><mml:mi>γ</mml:mi><mml:mn>300</mml:mn></mml:msub></mml:math>γ300, 28 for <mml:math><mml:msub><mml:mi>γ</mml:mi><mml:mrow><mml:mn>300</mml:mn><mml:mi>g</mml:mi></mml:mrow></mml:msub></mml:math>γ300g, 40 for <mml:math><mml:msub><mml:mi>γ</mml:mi><mml:mn>500</mml:mn></mml:msub></mml:math>γ500, 30 for <mml:math><mml:msub><mml:mi>γ</mml:mi><mml:mrow><mml:mn>500</mml:mn><mml:mi>g</mml:mi></mml:mrow></mml:msub></mml:math>γ500g, and 20 for <mml:math><mml:msub><mml:mi>γ</mml:mi><mml:mn>1300</mml:mn></mml:msub></mml:math>γ1300.

  2. Fig. 3.

    Stator kinetics. (A and B) The binding and unbinding rates of the stator units as a function of external viscous load on the motor (A) and single-stator force (B). All rates are calculated by fitting Eq. 3 to traces in Fig. 2C, with the exception of points outlined in cyan (SI Materials and Methods). (C and D) Dissociation constant, <mml:math><mml:msub><mml:mi>K</mml:mi><mml:mi>D</mml:mi></mml:msub></mml:math>KD, and lifetime of an individual stator in the motor complex, respectively, as a function of the average local force applied by a single stator to the rotor (and by symmetry to the PG layer). Points and error bars give averages and standard deviations, respectively.

  3. Fig. 4.

    Cartoon of a proposed catch-bond mechanism. The average force produced by the stator upon the rotor (blue arrow) stretches the stator anchoring point at the PG, inducing either conformational changes or a positional shift of the PGB within the PGB pocket that increase the bond strength and lifetime. The average force is higher for a larger viscous load (Right), with respect to a low viscous load (Left), as shown in Fig. 2A, and consistent with previously published torque–speed curves. IM, inner membrane.

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